1. Field of the Invention
The present invention relates to a cover glass that is subjected to shape-processing by etching and that is chemically-strengthened so as to have a compressive-stress layer on its principal surfaces, and to a method for producing the cover glass. The cover glass produced according to an embodiment of the present disclosure can be employed, for example, as a glass plate used as a component for protecting the display screen or the like of equipment such as a mobile phone, a personal digital assistant (PDA), a digital camera, or a flat panel display (FPD).
2. Background Art
Strengthened glasses produced by chemically strengthening glass substrates have been conventionally used, for example, as cover materials for protecting the liquid-crystal display screens etc. of equipment such as mobile phones, PDAs, digital cameras, and FPDs. The glass substrates are strengthened by an ion-exchange process.
In recent years, mobile phones and PDAs have tended to become thinner, more sophisticated in functionality, and more complicated in shape. Thus, strengthened glasses used as cover glasses in equipment such as mobile phones and PDAs are required to have formed therein recesses and/or holes with negative curvatures. Herein, what is meant by a negative curvature is that, if a point that is located on and moves along the contour of a given region keeps turning toward the right while the inner section of the region is always located on the left-hand side of the point, then the contour of said region is considered as having a negative curvature. On the other hand, the contour is considered as having a positive curvature if the point keeps turning toward the left as it moves along the contour while the inner section of the region is always located on the left-hand side of the point. The contour is considered as having zero curvature if the point keeps moving straight forward.
It is, however, difficult to subject a strengthened glass to outer-shape processing to form recesses or holes that include sections with negative curvatures, because the strengthened glass has a compressive-stress layer on its surface.
Meanwhile, JP-A-2009-167086 discloses a cover glass for mobile terminals that exhibits high strength even at a thin glass substrate and that can thus reduce the thickness of the device on which the cover glass is mounted.
The aforementioned cover glass is produced as follows. First, a resist pattern is formed on the principal surfaces of a plate-shaped glass substrate. Then, with the resist pattern serving as a mask, the glass substrate is etched with an etchant which consists of a mixed-acid aqueous solution containing hydrofluoric acid and at least one type of acid selected from sulfuric acid, nitric acid, hydrochloric acid, and hydrofluorosilicic acid, to thereby cut the glass substrate into a desired shape. Then, the etched glass substrate is subjected to chemical strengthening by an ion-exchange process.
With this method, it is possible to produce a cover glass having end surfaces that have a surface roughness of 10 nm or less in arithmetic mean roughness (Ra).
According to the aforementioned method, it is possible to produce a chemically-strengthened cover glass having complicated shapes with negative curvatures and/or through-holes by etching the plate-shaped glass substrate into predetermined shapes. In the above method, the processing time required for etching takes up a large proportion of the overall shape-processing step and has a huge impact on the cover-glass production efficiency. It is therefore important to shorten the processing time required for etching.
Further, in the chemical etching process of the aforementioned method, poorly-soluble chemical substances elute into the etchant, which contains hydrofluoric acid, and adhere to the glass substrate. This not only impairs the surface quality of the etched cover glass, but also inhibits the progress of the etching process if large amounts of chemical substances adhere to the glass surface, which may extend the processing time and impair the accuracy in shape.